Axial flow fan and housing for the same

ABSTRACT

An axial flow fan includes a housing that is formed by a process including injection molding includes a tapered surface at a frame portion thereof. One end of at least one rib is connected to an attachment portion which is a part of a stator portion of a motor portion, and the other end the rib is connected to a base portion protruding from the tapered surface. Two inclined surfaces connected to the tapered surface at the base portion and arranged in a circumferential direction with respect to a central axis are inclined such that the further a portion of the inclined surface from the central axis is, the further the portion is from a center of the rib. As a result, an air flow disturbance generated by the base portion between the rib and the frame portion is minimized, thereby reducing a noise generated due to the air flow disturbance.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a housing for an axial flow fan formed by a process including injection molding, and more particularly, to an axial flow fan utilizing the housing.

2. Description of the Related Art

Conventionally, a cooling fan such as an axial flow fan is used for cooling an interior portion of, or a specific member, in an electric component. The axial flow fan includes a motor portion including therein an impeller which rotates about a predetermined central axis, and a housing in which the motor portion is arranged. The housing includes a frame portion which surrounds a circumference of the motor portion and is centered about the central axis. Inside the frame portion, a stator portion of the motor is attached to an attachment portion which is located near one of the axial ends of the frame portion. Note, that the attachment portion can be a portion of the stator portion. The attachment portion is secured to the frame portion via a plurality of ribs extending in a direction that is perpendicular to the central axis toward an inner surface of the frame portion. In general, in a fan such as the one described above, the frame portion, the attachment portion and the plurality of ribs are integrally formed by a process including injection molding so as to reduce a production cost thereof.

Also, in recent years, in order to increase the cooling efficiency of the axial flow fan, a tapered surface is formed at a portion on the inner circumferential surface in which the rib is connected to the frame portion, wherein a distance between the tapered surface and the central axis gradually and continuously decreases toward a central area in an axial direction of the inner circumferential surface. However, such a housing is, due to structural restrictions imposed on a die assembly that is used for forming the housing, formed with a base portion protruding from the tapered surface. Normally, such a housing has a base portion which is larger in width in a circumferential direction than a rib.

In recent years, the demand for quieter electronic devices is increasing. Due to such demand, a quiet cooling fan used in the electronic device is also in demand. However, in the axial flow fan having the tapered surface on the housing thereof, an air flow generated by rotation of the impeller may be disturbed by the base portion inside the frame portion. Further, due to the air flow disturbance, noise will be generated when the fan is motion.

SUMMARY OF THE INVENTION

In order to overcome the problems described above, preferred embodiments of the present invention reduce the noise generated by the air flow disturbance in the axial flow fan having the housing in which the tapered surface is formed by the process including injection molding.

According to a preferred embodiment of the present invention, an axial flow fan includes a housing that is operable to suppress an air flow disturbance within the axial flow fan so as to reduce a noise generated by the air flow disturbance. The housing for the axial flow fan is preferably formed by a process including injection molding and preferably includes an attachment portion included in a stator portion of a motor having therein an impeller, a frame portion surrounding a space in which the motor is arranged coaxially with a central axis of the motor, and at least one rib connecting the attachment portion and the frame portion. An inner circumferential surface of the frame portion has at least one tapered surface wherein a distance between the central axis and the tapered surface gradually decreases from a portion of the tapered surface in contact with the at least one rib to a portion of the tapered surface toward a rotor portion of the motor. The frame portion includes at least one base portion to which the at least one rib is connected and which protrudes from the at least one tapered surface. The base portion has at a portion thereof that is in contact with the rib, a surface on either sides of the rib, and one of the surfaces is inclined such that the further a portion of the inclined surface from the central axis is, the further from a center line of the rib the portion of the inclined surface is.

Other features, elements, processes, steps, characteristics and advantages of the present invention will become more apparent from the following detailed description of preferred embodiments of the present invention with reference to the attached drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram showing a cross sectional view of an axial flow fan.

FIG. 2 is a diagram showing a perspective view of a housing.

FIG. 3 is a diagram showing a bottom plain view of the housing.

FIGS. 4 a, 4 b and 4 c are diagrams showing a cross sectional view of the housing.

FIG. 5 is a diagram showing an enlarged view of an area near a base portion.

FIG. 6 is a diagram showing a perspective view of the housing.

FIG. 7 is a diagram showing a top die assembly and a bottom die assembly.

FIG. 8 is a diagram showing a perspective view of a comparative exemplary housing.

FIG. 9 is a diagram showing a bottom plain view of the comparative exemplary housing.

FIG. 10 is a chart indicating a result of a measurement of a noise of an axial flow fan.

FIG. 11 is a chart indicating a result of a measurement of a noise of an axial flow fan.

FIG. 12 is a chart indicating a result of a measurement of a noise of an axial flow fan.

FIG. 13 is a diagram showing an example of a housing.

FIG. 14 is a diagram showing another example of a housing.

FIG. 15 is a diagram showing yet another example of a housing.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

According to preferred embodiments of the present invention, disturbance of an air flow within an axial flow fan is suppressed so as to reduce a noise which is generated by the air flow disturbance. The air flow disturbance is caused by a base portion provided between a rib of a housing, which has a tapered surface and is formed by a process including an injection molding, and an outer frame of the housing. In the description of the preferred embodiments of the present invention herein, words such as upper, lower, left, right, upward, downward, top, and bottom for explaining positional relationships between respective members and directions merely indicate positional relationships and directions in the drawings. Such words do not indicate positional relationships and directions of the members mounted in an actual device.

FIG. 1 is a longitudinal cross sectional view of an axial flow fan 1 according to a preferred embodiment of the present invention. The axial flow fan 1 preferably includes a motor portion 2 which rotates about a predetermined central axis J1, and a housing 3 in which the motor portion 2 is arranged. The motor portion 2 includes a rotor portion 21, and a stator portion 22. The stator portion 22 rotatably supports the rotor portion 21 via a bearing mechanism utilizing hydrodynamic pressure.

The rotor portion 21 includes an operative rotor portion 211 having a substantially cylindrical shape with its opening facing the stator portion 22, a ring shaped magnetic field magnet 212 which is magnetized to a plurality of magnetic poles and is centered around the central axis J1, wherein the magnetic field magnet 212 is affixed on an inner circumferential surface of the operative rotor portion 211. At a center of the operative rotor portion 211, an annular protruding portion 213 is provided. The annular protruding portion 213 is centered around the central axis J1 and protrudes toward the stator portion 22. One end (fixed end) of a shaft 214 is inserted into the annular protruding portion 213.

Also, the rotor portion 21 includes an impeller 215 which has a substantially cylindrical shape that is concentric with the central axis J1, in which the operative rotor portion 211 is inserted. The impeller includes a plurality of blades 21 on an outer circumferential surface thereof, and an annular shaped flange portion 217.

The stator portion 22 includes a substantially cylindrically shaped sleeve 221 which is preferably made of a porous metal material impregnated with oil, and a sleeve retaining portion 222 which is preferably made of a resin material. The sleeve 221 is inserted into the sleeve retaining portion 222 having the substantially cylindrical shape and a bottom. A non-fixed end of the shaft 214 is inserted into the sleeve 221. By virtue of the configuration, the shaft 214 is rotatably supported by the non-fixed end thereof through the oil and the sleeve 221. A thrust plate 223 facing the non-fixed end of the shaft 214 is provided at a bottom surface of the sleeve retaining portion 222. The thrust plate 223 is preferably made of a synthetic resin material having a low friction property, and supports the shaft 214. An armature 224 is arranged surrounding the sleeve retaining portion 222. The armature 224 is connected to an electric current supplying circuit 225. When the electric current controlled by the electric current supplying circuit 225 is supplied to the armature 224, a torque (a rotary force) is generated by a drive mechanism which includes the magnetic field magnet 212 and the armature 224 of the rotor portion 21 so as to rotate the rotor portion 21 with respect to the stator portion 22.

The housing 3 preferably includes an attachment portion 31 having a discoid shape that is centered around the central axis J1, and that is unitary and integral with the sleeve retaining portion 222 of the stator portion 22; a frame portion 32 having a ring shape that is centered around the central axis J1 so as to surround a circumference of the motor portion 2; and a plurality of ribs 33 each extending in a vertical manner with respect to the central axis J1 from the attachment portion 31 toward an inner surface of the frame portion 32. The attachment portion 31 is securely affixed to the frame portion 32 via the plurality of ribs 33. The inner surface of the frame portion 32 preferably includes a central circumferential surface 322 having a uniform distance between a mid point of the axial point in the axial direction and any mid point in the axial direction of the central circumferential surface 322 (that is, the central circumferential surface 322 is parallel or substantially parallel to the central axis J1); a tapered circumferential surface 321 whose distance to the central axis J1 increases toward a position where the rib 33 is arranged; a tapered circumferential surface 323 which is arranged at a portion near the central circumferential surface 322 opposite from the portion where the tapered circumferential surface 321 is arranged, and whose distance to the central axis J1 increases toward a portion farther away from the central circumferential surface 322. Note that, although a line defining the tapered surface 321 according to FIG. 1 which shows the cross sectional view of the housing 3 is a straight line, the defining line may be curved as shown in FIGS. 4 b and 4 c.

In the axial flow fan 1, in order to blow air, air will be taken into the housing 3, due to the rotation of the impeller 215, from one end in the axial direction of the housing 3 (e.g., an end of the circumferential surface 323), then the air will be moved in accordance with the direction of the rotation of the impeller 215, and then the air will be exhausted from the other end in the axial direction of the housing 3 (e.g., an end of the tapered surface 321). Due to the tapered surface 321 and the circumferential surface 323 formed inside the frame portion 32 of the axial flow fan 1, an efficiency of the air flow as described above will be improved.

FIG. 2 is a perspective view showing the housing 3. FIG. 3 is a bottom plain view showing the housing 3. FIGS. 4 a-4 c are cross sectional views showing the housing 3 as seen from a line A-A depicted in FIG. 3. It is to be appreciated that FIGS. 2 and 3 (and FIGS. 5, 6, 8, 9, 13 and 15, which will be described below) each show the housing 3 as seen from a side of the attachment portion 31, whereas FIGS. 2 and 4 a-4 c each show the axial flow fan or a portion thereof as seen upside down in the axial direction with respect to the axial flow fan shown in FIG. 1.

According to FIG. 3, a shape of the frame portion 32 (see FIG. 1) as seen from one end of the central axis J1 is preferably substantially rectangular. Note that the axial flow fan shown in FIG. 1 is shown in a diagonal perspective with respect to the substantially rectangular shape of the axial flow fan shown in FIG. 3.

Also, as described above, the frame portion 32 as shown in FIG. 4 a has, on the inner surface thereof, the tapered surface 321 in which the distance between the tapered surface 321 and the central axis J1 gradually and continuously decreases from the position where the ribs 33 are arranged toward the side of the rotor portion 21 of the motor portion 2 which will be arranged (that is, in a downward direction in accordance with FIG. 4 a). As shown in FIG. 3, an angle generated between the tapered surface 321 and the central axis J1 is the smallest at a portion of the tapered surface corresponding to a mid point of a side of the substantially rectangular shape of the frame portion 32 (e.g., according to FIG. 3, the angle is almost 0).

Hereinafter, a connection between each of the plurality of ribs 33 and the frame portion 32 will be described in detail. As shown in FIG. 2, a base portion 34 is provided for each of the plurality of ribs 33. The ribs 33 each are connected to the frame portion 32 through the base portion 34 which protrudes from the tapered surface 321 toward the rib 33. A surface 341 (hereinafter, the surface 341 will be referred to as a “connection surface 341”) of each of the base portions 34 facing the central axis J1 is, at any given point of the connection surface 341, equally spaced apart from the central axis J1 as the central circumferential surface 322 is from the central axis J1 (note that in FIG. 2, only one base portion 34 has an indication attached thereto). Also, the connection surface 341 has a surface that is parallel or substantially parallel to the central circumferential surface 322 with respect to the central axis J1. Note that in the housing 3, a border between the rib 33 and the base portion 34 is spaced an equal distance apart from the central axis J1 as the central circumferential surface 322 is from the central axis J1, that is, a portion of the rib 33 that is closer to the central axis J1 than the central circumferential surface 322 is to the central axis J1 is regarded as the rib 33 and a portion of the rib 33 closer to the frame portion 32 is regarded as the base portion 34.

FIG. 5 is an enlarged view of an area surrounding one of the base portions 34 in the housing 3. According to FIG. 5, a center line 331 is provided for each rib 33, wherein the center line is arranged along the direction of the rib 33. Also, in FIG. 5, the base portion 34 is indicated with parallel oblique lines. Two inclined surfaces 342 which face the central axis J1 in a circumferential direction (that is, the two surfaces are parallel or substantially parallel to the central circumferential surface 322 shown in FIG. 5) are provided to the base portion 34. The inclined surfaces 342 each are attached to the tapered surface 321 and are inclined such that the farther a portion of the inclined surface 342 is from the center line 331, the farther the portion is from the central axis J1.

As seen in FIG. 5, an outline of the inclined surface 342 of the base portion 34 is a gradual curve. To be more specific, an angle generated between a tangent line of the outline defining the inclined surface 342 and the center line 331 becomes greater as it is located further away from the central axis J1. Also, angles generated between the inclined surface 342 and the tapered surface 321 (that is, θ1 and θ2 in FIG. 5) at an area near a border between the inclined surface 342 and the tapered surface 321 are acute such that the inclined surface 342 continues smoothly to the tapered surface 321. Due to the aforementioned structure, each rib 33 is securely connected to the frame portion 32 via the base portion 34.

Next, hereinafter, forming of the base portion 34 during a manufacturing of the housing 3 will be described. Note that, in order to effectively show the forming of the base portion 34, a position of the base portion 34 in the housing 3 shown in FIG. 6 is slightly varied from those in FIGS. 2 and 3, however, the housing 3 in FIG. 6 is preferably manufactured in the same way as the housing 3 in FIGS. 2 and 3.

The housing 3 according to the present preferred embodiment is preferably made of a resin material and is manufactured by injection molding utilizing predetermined die assemblies. FIG. 7 is a diagram showing a top die assembly 91 and a bottom die assembly 92 used for forming the inner side of the frame portion 32 of the housing 3, the plurality of ribs 33 and the attachment portion 31. Note that in FIG. 7, an indication of elements that are not directly related to forming of the base portion 34 of the housing 3 are omitted.

In FIG. 7, an area 911 indicated with parallel oblique lines in the top die assembly 91 corresponds to a base portion 342 a indicated with parallel oblique lines in FIG. 6; and an area 921 indicated with parallel oblique lines in the bottom die assembly 92 corresponds to the connection surface 341 indicated with parallel oblique lines in FIG. 6.

According to the manufacturing method of the housing 3 as shown in FIG. 6, the top die assembly 91 and the bottom die assembly 92 are joined together and placed in a space within another die assembly which is used for forming the exterior of the frame portion 32. The resin material is injected in a space generated between the aforementioned top and bottom die assemblies 91 and 92. The die assembly for forming the exterior of the frame portion 32 will be detached after the resin material is cooled and hardened. Then the top die assembly 91 and the bottom die assembly 92 will be detached from one another in the axial direction. By this, the housing 3 is formed while the inner side of the frame portion (e.g., the tapered surface 321, central circumferential surface 322, the circumferential surface and the base portion 34), the plurality of ribs 33, and the attachment portion 31 are simultaneously formed. Note that a parting line generated inside the housing 3 between the top die assembly 91 and the bottom die assembly 92 will be a line that defines the tapered surface 321 and the central circumferential surface 322 except for the portion where the base portion 34 is arranged. Also note that, as is clear from a shape of the area 921 of the bottom die assembly 92 shown in FIG. 7, a cross section of each rib 33 in the housing 3 preferably has a substantially triangular shape.

Hereinafter, how the base portion 34 in the housing 3 is formed will be described with reference to FIGS. 6 and 7. When the housing 3 is formed by the injection molding, the plurality of ribs 33 are formed in the space generated by assembling the top die assembly 91 and the bottom die assembly 92 together in the axial direction. As a result, in order to separate the top die assembly 91 and the bottom die assembly 92 by effectively removing one from the other in the axial direction, the tapered surface 321 which extends further outwardly than the central circumferential surface 322 is to be formed by the top die assembly 91. However, a portion (hereinafter, referred to as a focus portion) of the tapered surface 321 between the rib 33 and the central circumferential surface 322 cannot be formed by the top die assembly 91 because of the rib 33. Therefore, the focus portion needs to be formed by a portion of the bottom die assembly 92. When the bottom die assembly 92 forms the central circumferential surface 322 which has a predetermined distance to the central axis J1, the focus portion having an equal distance to the central axis J1 (that is, the distance between the central circumferential surface 322 and the central axis J1 equals the distance between the focus portion and the central axis J1) will also be formed, and therefore, the base portion 34 which protrudes from the tapered surface 321 will be formed as a consequence of the manufacturing method.

On the other hand, it is a prerequisite that a surface of each of two die assemblies, in contact with another surface, has a predetermined size and dimension in order to maintain durability of the connection between the top and the bottom die assemblies. Therefore, as for the top die assembly 91 and the bottom die assembly 92, a portion 923 (hereinafter, referred to as a “rib forming portion 923”; there is only one indication of the rib forming portion 923 in FIG. 7) of the bottom die assembly 92 in which the rib 33 will be formed has two contact surfaces 922 (indicated with parallel oblique lines in FIG. 7) having a predetermined width therebetween, and the top die assembly 91 has a portion corresponding to the rib forming portion 923. Since an outward facing end surface of the rib forming portion 923 is the area 921 for forming the connection surface 341 at the base portion 34, the connection surface 341 will be in the circumferential direction as wide as the area 921. That is, the outward facing end surface is wider than the rib 33 in the circumferential direction as much as the width of the two contact surfaces 922.

It is to be appreciated that the bottom die assembly 92 is, in actuality, structured such that a diameter of the central circumferential surface 322 decreases slightly and continuously toward the tapered surface 321. By virtue of such a configuration, the bottom die assembly 92 can be separated effectively from the top die assembly 91 after the injection molding is completed.

Next, a conventional housing which is formed by an injection molding and which may be comparable to the housing 3 of the present preferred embodiment will be described. FIG. 8 is a diagram showing a perspective view of a housing 8 which will be compared to the housing 3. FIG. 9 is a diagram showing a bottom plain view of the housing 8. An inner surface of the frame portion 82 of the housing 8 has a tapered surface 821 whose distance to a central axis J2 gradually and continuously increases from a central circumferential surface 822 (which is equivalent to the central circumferential surface 322) toward a position where a plurality of ribs 83 are arranged. Also, since the housing 8 is formed by a process including the injection molding, a base portion 84 for each rib 83 will be formed consequently. However, the base portion 84 of the housing 8 makes contact with the tapered surface 821 of the frame portion 82 while the base portion 84 has two surfaces which are parallel or substantially parallel to one another sandwiching the central axis J2 in a circumferential direction and which are also parallel or substantially parallel to a center line 831 (see FIG. 9).

Hereinafter, a result of a measurement of a noise generated by the axial flow fan 1 having therein the housing 3 will be compared to that generated by an axial flow fan having therein the housing 8. Preferably, a motor portion of each axial flow fan rotates at 3,200 rpm and has an impeller having 7 blades, and therefore, a first order component occurs at a frequency of 373 Hz ((3200/60)×7), for example.

FIG. 10 shows a result of a measurement of a noise generated when the axial flow fan 1 having therein the housing 3 is driven. FIG. 11 shows a result of a measurement of a noise generated when the axial flow fan having therein the housing 8 is driven. FIG. 12 shows an enlarged view of a portion of FIG. 10 and FIG. 11 depicting a specific range of frequency. In FIG. 12, the measurement result of the axial flow fan 1 is indicated by a reference numeral 71 and that of the other axial flow fan having the housing 8 is indicated by a reference numeral 72.

According to FIG. 12, a large difference in acoustic pressure is found between 71 and 72 approximately at a second order component (746 Hz) and at a third order component (1119 Hz). A noise level of the acoustic pressure of the axial flow fan 1 having the housing 3 is about 22.4 dB(A), and a noise level of the acoustic pressure of the axial flow fan having the housing 8 is about 23.0 dB(A). That is, the axial flow fan 1 having the housing 3 has a smaller acoustic pressure compared with the axial flow fan having the housing 8 by about 0.6 dB(A).

As described above, the housing 3 of the axial flow fan is formed by a process including injection molding and by the die assemblies. The housing 3 has the frame portion 32 at which the base portion 34 is provided for each rib 33 which is used for securing, via the base portion 34, the attachment portion 31. Also, each rib 33 has a pair of inclined surfaces 342 whose distance to the central axis J1 increases as the distance between the portion of the inclined surface 342 and the center line 331 increases. As a result of such a configuration, the axial flow fan 1 of the present preferred embodiment having therein the housing 3 which is formed by the process including injection molding and which includes therein at least one tapered surface increases the cooling efficiency. Also, the axial flow fan 1 of various preferred embodiments of the present invention having therein the housing 3 is operable to suppress the air flow disturbance caused by the base portion 34 which is disposed between the rib 33 and the frame portion 32, thereby reducing the noise generated due to the air flow disturbance.

FIG. 13 is a diagram showing a bottom plain view of the housing 3 according to another preferred embodiment of the present invention. According to the frame portion 32 of the housing 3 in FIG. 13 is, when viewed in the axial direction, substantially rectangular in shape. In FIG. 13, an area 321 a (hereinafter, referred to as a “specific tapered surface 321 a”) is provided to the tapered surface 321. The specific tapered surface 321 a is a portion of the tapered surface 321 nearest to the frame portion 32, wherein an angle generated by the central axis J1 and the specific tapered surface is smaller than that generated by the central axis J1 and the tapered surface 321, and the distance between the central axis J1 and specific tapered 321 a is shorter than that between the central axis J1 and the rest of the tapered surface 321.

According to the housing 3 as shown in FIG. 13, the base portion 34 is arranged for each rib 33 axially above the specific tapered surface 321 a. The base portion 34 is arranged so as to connect each rib 33 and the frame portion 32. By virtue of the configuration, a height of the base portion 34 (i.e., a distance between the tapered surface 321 and the central axis J1) will be lower than when the base portion 34 is arranged at an area other than axially above the specific tapered surface 321 a. Therefore, the housing 3, which is formed by the process including the injection molding, as shown in FIG. 13 is operable to suppress the air flow disturbance caused by the base portion 34, thereby reducing the noise which is generated by the air flow disturbance.

FIG. 14 is a diagram showing a bottom plain view of another example of a housing. FIG. 14 shows an enlarged view of a portion where a rib 33 and the base portion 34 are connected to one another in the another example of the housing. Note that the base portion 34 is indicated with oblique parallel lines in FIG. 14.

According to FIG. 14, at the area in which the rib 33 and the frame portion 32 of the housing 3 are connected to one another a width of the rib 33 gradually increases toward the housing 3, and the surface of the rib 33 and the inclined surface 342 of the base portion 34 are connected. Also, according to the cross sectional view of the housing 3, curved lines connecting the rib 33 and the tapered surface 321 via the base portion 34 are formed. By this, the axial flow fan 1 according to the present preferred embodiment of the present invention, the air flow disturbance generated a central axis J1 side of the base portion 34 in the housing 3 shown in FIG. 3 will be reduced, thereby suppressing the noise generated which is due to the air flow disturbance. Note that, since the die assembly used to form the housing 3 shown in FIG. 14 is structured in a more complicated manner than that used to form the housing 3 shown in FIG. 3 is, in order to have a lower manufacturing cost for the die assembly, the housing 3 as shown in FIG. 3 may be preferred. Also note that, the rib 33 and the base portion 34 as shown in FIG. 14 may be applied to the housing 3 as shown in FIG. 13.

While the preferred embodiments of the present invention have been described in detail, the foregoing description is in all aspects illustrative and not restrictive. It is understood that numerous other modifications and variations can be devised without departing from the scope of the present invention.

For example, although according to the housing 3 having the inclined surface 342 formed therein, the noise generated by the air flow disturbance will be minimized, the comparative exemplary housing 8 shown in FIG. 9 may have a pair of inclined surfaces (hereinafter, referred to as “specific aspect”) which are formed by grinding the base portion 84 and which are arranged in the circumferential direction with respect to the central axis J2. The specific aspect is such that the specific surface 84 makes contact with the tapered surface 821, and the further the portion of the specific aspect from the central axis J2 is, the further it will be from the center line 831.

Further, when the inclined surface 342 is formed on one of the specific aspects of the base portion 34, the air flow disturbance will be suppressed to a certain extent. That is, it is important that at least one of the two specific aspects includes the inclined surface 342. Note that when only one of the specific aspects of each rib 33 includes the inclined surface 342, it is important that one specific aspect arranged on a same side with respect to each rib 33 is to include the inclined surface 342 so as to reduce the air flow disturbance.

Further, it is to be noted that only a certain portion of the specific aspect can be the inclined surface 342 in order to achieve the advantages described above. More particularly, it is preferable that the inclined surface be disposed at a portion of the specific aspect connected to the rib 33. Also, in order to effectively suppress the air flow disturbance when the inclined surface is to be disposed as mentioned above, it is preferable that, if the specific aspect extends in the same direction as that the rib 33 extends (as shown in FIG. 9), an area of the specific aspect equivalent to a half of the specific aspect will be the inclined surface.

It is to be noted that in the housing 3, the inclined surface may have any shape as long as the inclined surface is shaped such that the further a portion of the inclined surface from the central axis J1 is, the further from the center line of the rib 33 it will be. For example, as shown in FIG. 15, the rib 33 and the base portion 34 may have a same width at the joint portion therebetween, wherein a pair of continuous and gradual inclined surfaces 342 maybe provided. However, in order to suppress the air flow disturbance in the axial flow fan 1, the inclined surface 342 should preferably be formed such that the further the portion of the inclined surface 342 from the central axis J1 is, the greater the angle generated between the inclined surface 342 and the center line of the rib 33 is, and that, when seen from one end of the axial direction, the edge of the inclined surface 342 defines a curved line.

Further, although the axial flow fan 1 according to FIG. 1 preferably has the stator portion 22 including the attachment portion 31 and the sleeve retaining portion 222 as one seamless unitary component, the sleeve retaining portion 222 and the attachment portion 31 can be formed separately from one another, wherein the attachment portion 31 is attached to the stator portion 22.

Further, although the housing 3 according to FIG. 13 preferably has a substantially rectangular shape when seen from one end of the axial direction, the frame portion of the housing can have any shape. For example, the frame may have a substantially round shape in which a portion of the frame includes a straight line. When the frame portion takes the substantially round shape, a portion of the inner surface of the frame portion which generates the smallest angle with respect to the central axis J1 maybe the specific tapered surface upon which in the axial direction the base portion can be provided. By virtue of such configuration, the air flow disturbance generated by the base portion may be minimized.

Furthermore, note that in housing 3, the ribs 33 which connect the frame portion 32 to the attachment portion 31 need not extend perpendicularly to the central axis J1. The rib 33 may be tilted with respect to the perpendicular line to the axial direction.

While preferred embodiments of the present invention have been described above, it is to be understood that variations and modifications will be apparent to those skilled in the art without departing the scope and spirit of the present invention. The scope of the present invention, therefore, is to be determined solely by the following claims. 

1. An axial flow fan having an injection molded housing, comprising: an attachment portion arranged to be provided in a stator portion of a motor having therein an impeller; a frame portion surrounding a space in which the motor is arranged and coaxial with a central axis of the motor; and at least one rib connecting the attachment portion and the frame portion; wherein an inner circumferential surface of the frame portion has at least one tapered surface wherein a distance between the central axis and the tapered surface gradually decreases from a portion of the tapered surface in contact with the at least one rib to a portion of the tapered surface extending toward a rotor portion of the motor; the frame portion includes at least one base portion to which the at least one rib is connected and which protrudes from the at least one tapered surface; the base portion has at the portion thereof in contact with the rib a surface on either side of the at least one rib, and one of the surfaces is inclined such that the further a portion of the inclined surface from the central axis is, the further from a center line of the rib the portion of the inclined surface is.
 2. The axial flow fan having the housing according to claim 1, wherein the surfaces on either side of the at least one rib are inclined such that the further a portion of each of the inclined surface from the central axis is, the further from the center line of the rib the portion of the inclined surface is.
 3. The axial flow fan having the housing according to claim 1, wherein the further the distance between a portion of the inclined surface and the central axis is, the greater an angle generated between the portion of the inclined surface and a center line of the rib becomes.
 4. The axial flow fan having the housing according to claim 1, wherein the attachment portion is secured to the frame portion by a plurality of the ribs, and one of each pair of the surfaces disposed on a same side of each base portion of each rib includes the inclined surface.
 5. The axial flow fan having the housing according to claim 1, wherein the inclined surface is injection molded.
 6. The axial flow fan having the housing according to claim 5, wherein an outline of the frame has a shape including a straight line when viewed from an angle parallel to the central axis, a portion of the inner surface of the frame portion nearest to the straight line is a specific tapered surface at which an angle generated between the central axis and the specific tapered surface is smallest among any other portion of the inner surface of the frame portion, and the base portion is disposed on the specific tapered surface.
 7. The axial flow fan having the housing according to claim 1, wherein, a portion of the rib and a portion of the base portion are connected to one another, and the further a portion of the rib from the central axis is, the wider the rib is.
 8. An axial flow fan comprising: a motor, including: a rotor portion including an impeller; a stator portion; and an injection molded housing, including: an attachment portion provided in the stator portion; a frame portion surrounding a space in which the motor is arranged and coaxial with a central axis of the motor; and at least one rib connecting the attachment portion and the frame portion; wherein an inner circumferential surface of the frame portion has at least one tapered surface wherein a distance between the central axis and the tapered surface gradually decreases from a portion of the tapered surface in contact with the at least one rib to a portion of the tapered surface extending toward a rotor portion of the motor; the frame portion includes at least one base portion to which the at least one rib is connected and which protrudes from the at least one tapered surface; the base portion has at the portion thereof in contact with the rib a surface on either side of the at least one rib, and one of the surfaces is inclined such that the further a portion of the inclined surface from the central axis is, the further from a center line of the rib the portion of the inclined surface is. 